Solve the system of equations. $\begin{aligned} & -4x+7y = 20 \\\\ & y=3x+15 \end{aligned}$ $ x=$
Answer: We are given that $ y = {3x+15}$. Let's substitute this expression into the first equation and solve for $x$ as follows: $\begin{aligned} -4x+7{y}&=20\\\\ -4x+7\cdot({3x+15})&=20\\\\ -4x+21x+105& = 20\\\\ 17x&=-85\\\\ x&=-5 \end{aligned}$ Since we now know that $ x={-5}$, we can substitute this value into the second equation to solve for $y$ as follows: $\begin{aligned} y &= 3\cdot {x}+15 \\\\ y&=3\cdot({-5})+15\\\\ y&=0 \end{aligned}$ This is the solution of the system: $\begin{aligned} &x = -5 \\\\ &y=0 \end{aligned}$